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Wideband Single-Crystal Transducer for Bone Characterization

Saturday, 01 December 2012

These transducers have
uses in medical ultrasound
imaging and room-temperature
ultrasonic
flow meters.

The microgravity conditions of space
travel result in unique physiological
demands on the human body. In particular,
the absence of the continual mechanical
stresses on the skeletal system that
are present on Earth cause the bones to
decalcify. Trabecular structure decreases
in thickness and increases in spacing,
resulting in decreased bone strength and
increased risk of injury. Thus, monitoring
bone health is a high priority for
long-term space travel. A single probe
covering all frequency bands of interest
would be ideal for such measurements,
and this would also minimize storage
space and eliminate the complexity of
integrating multiple probes.

This invention is an ultrasound transducer
for the structural characterization
of bone. Such characterization measures
features of reflected and transmitted
ultrasound signals, and correlates these
signals with bone structure metrics such
as bone mineral density, trabecular spacing,
and thickness, etc. The techniques
used to determine these various metrics
require measurements over a broad
range of ultrasound frequencies, and
therefore, complete characterization
requires the use of several narrowband
transducers.

This is a single transducer capable of
making these measurements in all the
required frequency bands. The device
achieves this capability through a unique
combination of a broadband piezoelectric material; a design incorporating multiple
resonator sizes with distinct, overlapping
frequency spectra; and a micromachining
process for producing the multiple-
resonator pattern with common electrode
surfaces between the resonators.

This device consists of a pattern of resonator
bars with common electrodes that
is wrapped around a central mandrel
such that the radiating faces of the resonators
are coplanar and can be simultaneously
applied to the sample to be measured.
The device operates as both a
source and receiver of acoustic energy. It
is operated by connection to an electronic
system capable of both providing an
excitation signal to the transducer and
amplifying the signal received from the
transducer. The excitation signal may be
either a wide-bandwidth signal to excite
the transducer across its entire operational
spectrum, or a narrow-bandwidth
signal optimized for a particular measurement
technique. The transducer face is
applied to the skin covering the bone to
be characterized, and may be operated in
through-transmission mode using two
transducers, or in pulse-echo mode.

The transducer is a unique combination
of material, design, and fabrication
technique. It is based on single-crystal
lead magnesium niobate lead titanate
(PMN-PT) piezoelectric material. As compared
to the commonly used piezoceramics,
this piezocrystal has superior piezoelectric
and elastic properties, which
results in devices with superior bandwidth,
source level, and power requirements.
This design necessitates a single
resonant frequency. However, by operating
in a transverse length-extensional
mode, with the electric field applied
orthogonally to the extensional direction,
resonators of different sizes can share
common electrodes, resulting in a multiply-
resonant structure. With carefully
sized resonators, and the superior bandwidth
of piezocrystal, the resonances can
be made to overlap to form a smooth,
wide-bandwidth characteristic.

This work was done by Yu Liang and Kevin
Snook of TRS Technologies, Inc. for Glenn
Research Center.

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